I guess the first question (which isn't clear from your post) is if you are writing new data to the pool after adding the drives. Are the LUNs you are writing to thick or thin? Did you create new LUNs after adding the drives to the pool?

The link that SKT provided explains VNX storage pools which are different from VNXe. If you accurately wrote in your original question that you had a new VNXe (not VNX) then you may find this other blog post more helpful.. It is not my post, I just found it through a Google search.